Method for crosstalk reduction between tracks on a recording medium, recording device, playback device and recording medium

ABSTRACT

In order to reduce the cross talk between data recorded in adjacent tracks on a record carrier the encoding of the data stream into code words is controlled using control points. The code words in a first track are altered by selecting that value of the control point that results in code words that differ in as many bit positions as possible from the corresponding bit positions in a second track, where the first track and second track are both adjacent to the same third track. Having opposite bit values in corresponding bit positions on the first and second track results in the lowest contribution of these bit positions to the code words stored in the third track.

This invention relates to a method for encoding a stream of input wordsinto a stream of code words using a channel code for storage of thestream of code words on a recording medium comprising tracks for storageof the stream code words comprising the steps of:

-   -   encoding the stream of input words into the stream of code        words, to a recording device for recoding data on a record        carrier, to a record carrier, to a playback device and to an        encoder.

In order to keep the crosstalk between neighboring tracks on a recordcarrier at an acceptable level the tracks are positioned relatively farapart. The further apart the lower the crosstalk so the crosstalkdefines a minimum distance between the track, the minimum track pitch.The problem associated with this minimum track pitch is that it resultsin a maximum recording density (bits per square centimeter) whichimposes a maximum recording capacity on a record carrier with a givenphysical size.

It is an objective of this invention to overcome this problem byproviding a method which reduces the minimum track pitch withoutincreasing the cross talk between neighboring tracks.

In order to achieve this objective the method is characterized in thatthe method comprises the following steps

-   -   determining a control point in the data stream of input words or        the stream of code words where the data stream of input words or        the stream of code words can be altered by an alteration.    -   for each alteration of a group of N possible alterations        determining, between a group of code words in a first track and        a group of code words in a second track which is adjacent to a        third track which is adjacent to the first track, a crosstalk        value representing the cross talk affecting the third track        corresponding to the alteration.    -   Selecting an optimum alteration, where the optimum alteration is        that alteration from the group of N alterations which has a        lowest cross talk value,    -   Altering the data stream using the optimum alteration.

A control point is a point in the data stream where the subsequent partof the data stream can be influenced. By calculating for each of theoptions at the control point the resulting subsequent part of the datastream and selecting that option at the control point that results inthe lowest crosstalk at a given point on the record carrier the crosstalk can be lowered. This lower crosstalk can than be traded in, in theregular fashion, for a reduced track pitch. Thus by applying the methodaccording to the invention the improvement in crosstalk and resultingimprovement in signal to noise ratio allows other parameters affectingthe signal to noise ratio to be chosen such that the improved signal tonoise ratio is worsened again to the minimum acceptable level. Not onlytrack pitch can be reduced when the method according to the invention isapplied but also recording systems with worse signal to noise ratios forinstance as a result of recording media with a reduced signal to noiseratio.

It is of course also possible to use the method to increase the signalto noise ratio to obtain a lower bit error rate during reading andwriting on the storage medium by simply leaving all other parametersaffecting the signal to noise ratio unchanged.

An embodiment of the method is characterized in that in that N=2. Bylimiting N to 2 a single bit or a choice of only 2 options suffices tocontrol the crosstalk. This simplifies the calculations to be performedby the recording device and the playback device.

An embodiment of the method is characterized in that the control pointis a bit insertion point. By inserting a bit into the data stream atpredefined places in order to allow the playback device to distinguishthe inserted bit, the encoding of the subsequent data stream can beinfluenced. When a bit with a value of ‘0’ is inserted at the bitinsertion point a different encoded data stream will result then when abit with a value of ‘1’ is inserted. After calculating the encoded datastream, the bit corresponding to the calculated data stream whichresults in the lowest crosstalk value is inserted into the data streamat the bit insertion point and encoded. The calculation can be executedfor the subsequent data stream up to the next bit insertion point sothat the sections of the data stream between the bit insertion pointsare each individually optimized for crosstalk.

An embodiment of the method is characterized in that the control pointis a code word replacement point.

Instead of a bit insertion point a code replacement word can be chosen.

Many codes have code words that can never occur when encoding datastreams. Such a code word can be used to change the crosstalk. A tableis created that is known to both the recording device and the playbackdevice. When the recording device encounters a code word from the tableit has the option of leaving the code word in the encoded data stream orto replace the code word with the replacement code word according to thetable. By choosing the replacement code words from the set of code wordsthat can never occur the playback device is able to distinguish thereplacement code word from the other code words in the encoded datastream and replace the replacement code word with the corresponding codeword of the table. The choice of replacement code word can be madedependant on the state of the coder, comparable to the method used inEFM-plus encoding and decoding. The method of altering a data streamusing replacement code words is disclosed in patent application EP02076424.7. The recording device chooses whether to replace the codeword by the replacement code word depending on the calculated effect onthe crosstalk. Because of the NRZI encoder used to encode the encodeddata stream into NRZI format suitable for the recording medium thereplacement code word can affect the subsequent NRZI encoded data streamby differing in the number of ‘1’ bits by an odd number compared to thecode word to be replaced. Changing the number of ‘1’ bits from even toodd or from odd to even means that all subsequently NRZI encoded bitscoming out of the NRZI coder will change polarity because a ‘1’ goinginto the NRZI coder means a change in level at the output of the NRZIcoder.

A further embodiment of the method is characterized in that a crosstalkvalue is determined calculating a running digital sum value of anexclusive NOR operation performed bitwise on the group of code words inthe first track and the group of code words in the second track.

The crosstalk between tracks is lowest when the bits in the first trackhave the opposite polarity of the bits in the second track. Since theperfect situation cannot be obtained because in that case the contentsof one track would dictate that the contents of the second track must bethe precise inverse of the contents of the first track, the methoddetermines a digital sum value to obtain an indication of the amount ofbits located close to each other but on the second track which are ofopposite polarity. The exclusive NOR operation determines whether bitson the first track are the opposite polarity of the bits on the secondtrack which are located in corresponding bit positions. In this way abit wise comparison of a group of bits in the first track to a group ofbits in the second track is achieved. If the digital sum is low thepolarity of the group of bits in the first track differs substantiallyfrom the polarity of the group of bits in the second track, i.e.crosstalk is low. If the digital sum is high the polarity of the groupof bits in the first track resembles the polarity of the group of bitsin the second track, i.e. crosstalk is high.

A further embodiment of the method is characterized in that the group ofcode words in the first track is limited to a section of the first trackand that the group of code words in the second track is limited to asection of the second track and that the section of the first track isaligned perpendicular to a reading direction of the first track with thesection of the second track.

Instead of calculating the digital sum for complete tracks thecalculation can also be performed for only a section of tracks that arealigned perpendicular to the reading direction. This requires morecontrol points to be used but this results in improved control over thecrosstalk in smaller areas allowing a better optimization. It is ofcourse imperative to have the sections of the tracks involved in thebitwise exclusive NOR operation exactly aligned, i.e. the start of thesection on the first track must be aligned with the start of thecorresponding section on the second track and the end of the section onthe first track must be aligned with the end of the correspondingsection on the second track.

The data can be stored in the tracks in several ways without affectingthe effectiveness of the invention:

-   -   Data represented by pits on an optical recording medium    -   Data represented by modulation of the track position on an        optical recording medium    -   Data represented by magnetic regions on a optical/magnetic or        magnetic recoding medium        Wherever the data is represented by physical differences in the        recording medium and the data is read out in a way that bits in        close proximity to the bit being read can increase the read or        write noise level by crosstalk the invention can be applied.

The invention can also be applied to the parallel transmission of datathrough transmission channels that exhibit cross talk. When thetransmission is coordinated the encoding of the data can include controlpoints where the encoding of the data on a first channel can be alteredaccording to the invention compared to the data in a second channel, inorder to reduce the noise level contribution of the first and secondchannel to a third channel.

The invention will now be described based on figures.

In order to distinguish more clearly between the overall encoder and theencoders that are comprised within the overall encoder, the encoderscomprised in the overall encoder are called ‘coder’ while the overallencoder is referred to as ‘encoder’.

FIG. 1A shows a section of adjacent tracks

FIG. 1B illustrates the concept of crosstalk and corresponding bitpositions.

FIG. 1C illustrates the concept of crosstalk and corresponding bitpositions in relation to another reading spot shape

FIG. 2 shows a record carrier in the shape of a disc comprisingconcentric tracks

FIG. 3 shows a record carrier in the shape of a disc comprising aspiraling track.

FIG. 4 shows an encoder for encoding the data to be recorded in thetracks

FIG. 5 shows a further encoder for encoding the data to be recorded inthe tracks

FIG. 6 shows a flow chart of a software implementation of the crosstalkreduction.

FIG. 7 shows a graph of the digital sum value indicating the level ofcrosstalk

FIG. 8 shows a recording device comprising the invention.

FIG. 9 shows a playback device.

FIG. 1A shows a section of adjacent tracks. A first track 1 and a secondtrack 2 are located adjacent to a third track 3.

In order to reduce the crosstalk in the third track 3 the bit values ofthe bits in the adjacent tracks 1,2 should have the opposite polarities.In the present example the bit values in the second track 2 differ fromthe bit values in the first track 1 in all positions except the eighthposition P8, the eleventh position P11 and the twelfth position P12. Itis clear that bits of the second track 2 cannot have the exact oppositebit values of the bit values of the first track 1 because otherwise noinformation could be recorded in the second track 2. The bit positionsof the third track 3 show a bit value of ‘don't care’ because the actualstored value is of no importance to the invention. The measures of theinvention are taken in the adjacent tracks 1,2 only. It is the bitvalues on the adjacent tracks 1, 2 that cause the cross talk. Bybalancing the crosstalk contribution of a ‘1’ on the first track 1 by a‘0’ on the corresponding position on the second track 2, and of a ‘0’ onthe first track 1 by a ‘1’ on the second track 2 the overall influenceof the adjacent tracks 1, 2 on the data stored in the third track 3 isreduced.

Because the bit values of the first track 1 and the second track 2 inposition P8 are both ‘0’ the bit values are not opposite and contributeto the cross talk in the same way, thus adding to the crosstalk andincreasing the noise level of the data bit in position p8 in the thirdtrack 3. The same is true for the twelfth position P12 where the bitvalue of both the first track 1 and the second track 2 are ‘1’, thus notbalancing each other but contributing to the crosstalk in the same way.

The data stored in the second track 2 should be in as many positions aspossible the exact opposite of the data stored in the first track 1 incorresponding bit positions.

FIG. 1B illustrates the concept of crosstalk and what corresponding bitpositions are.

Shown are three tracks 1, 2, 3 where data is stored. The circleindicates the size of the reading spot 4B. When the track pitch isreduced the data bits 4C, 4D comprised in the third track's 3neighboring tracks 1,2 are included in the area covered by the reading(or writing) spot 4B and thus contribute to noise level when reading thedata bit 4A of the third track. These included data bits 4C, 4D aredescribed in this document as being in corresponding bit positions onthe first track 1 and on the second track 2.

The reading direction of each track is in the direction of theelongation of the track.

With the reading spot 4B as shown in FIG. 1B the data bits 4C, 4D thatare included in the reading (or writing) spot are aligned perpendicularto the reading direction with the data bit 4A to be read out.

FIG. 1C illustrates the concept of crosstalk and what corresponding bitpositions are in relation to another reading spot shape.

Shown are three tracks 1, 2, 3 where data is stored. The tilted ovalindicates the size of the reading spot 5B. When the track pitch isreduced the data bits 5E, 5F comprised in the third track's 3neighboring tracks 1,2 are included in the area covered by the reading(or writing) spot 5B and thus contribute to noise level when reading thedata bit 5A of the third track. It is clear that depending on the shapeof the read-out or write spot the data bits on the neighboring tracksthat affect the crosstalk can have different positions relative to thedata bit to be read-out or written. Even though the data bits 5E, 5Fwhich contribute to the noise level of the data bit to be read-out areno longer perpendicular aligned with the data bit to be read out thecontributing data bits 5E, 5F are still considered to be atcorresponding bit positions on the first track 1 and the second track 2.The data bits 5C, 5D that would be contributing to the noise level ifthe read-out spot 5B would be circular are in the case of the elongatedoval shape of FIG. 1C no longer contributing to the noise level and arehence no longer considered to be on corresponding bit positions.

FIG. 1C also illustrates that due to the shape of the read-out spot ofthe write spot multiple bits in the first track 1 and second track 2 canbe comprised in the spot and each bit can be comprised in the spotbetween 0 and 100%. Consequently it is advantageous to apply not onlythe crosstalk determination to the bits in the first track 1 and thesecond track 2 that are comprised in the spot to the highest percentage,but also bits directly adjacent to these bits. In order to reflect theirlower contribution to the crosstalk a weighing function is applied. Theweighing function can reflect the physical distance between the bitscausing the crosstalk and the affected bit since crosstalk is a directfunction of distance. The weighing can also be based on the physicalshape of the read-out spot, the write spot or the shape of the pits.

FIG. 2 shows a record carrier with concentric tracks.

Because of the concentric tracks each track holds a slightly differentamount of data compared to the adjacent tracks. This would theoreticallypose a problem because the tracks that are supposed to be each other'sopposite as much as possible hold different amounts of data. Becausethere are many tracks and the tracks are located very closely togetherthe difference in the amount of data between a first track 21 and asecond track 22 is very small.

When observing the tracks locally, for instance in the pie section 24indicated, the curvature is very small because of the radius of thetrack and the size of the pits that the tracks can be considered to bestraight and to run parallel for that section of the tracks that isrelevant for the crosstalk.

It is furthermore not required to obtain exact opposite polarity of thetracks for all positions since information must be stored which resultsin differences between the tracks anyway. It is consequently no problemto have different amounts of data in the tracks since it is the overallreduction in crosstalk by striving to opposite polarity for as many bitpositions as possible that will contribute to a lower Bit Error Rate inthe third track 23.

FIG. 3 shows a record carrier with a track spiraling outward. Whenobserving the tracks locally, for instance in the pie section 34indicated, the curvature is very small because of the radius of thetrack and the close proximity of the tracks compared to the radius ofthe tracks. The sections of the tracks that are adjacent to each othercan be considered to be sections of adjacent concentric tracks asdiscussed in FIG. 2 instead of sections of a spiraling track. Thediscussion of FIG. 2 is thus also valid for the case where there is asingle spiraling track, spiraling inward or outward.

FIG. 4 shows an encoder 40 comprising an coder 41. The data to berecorded on the record carrier is presented to the input of the coder41, is encoded by the coder 41 and the encoded data is provided at theoutput of the coder 41. From the output of the coder 41 the encoded datais passed to the input of the first bit insertion means 42A and to theinput of the second bit insertion means 42B. The first bit insertion 42Ameans inserts a ‘0’ bit at predetermined control points in the encodeddata stream. The second bit insertion means 42B inserts a ‘1’ bit atpredetermined control points in the encoded data stream. The first bitinsertion means 42A provides the encoded data stream comprising ‘0’ bitsat the predetermined control points to the first NRZI coder 43A whichencodes the data and provides the resulting NRZI encoded data based onthe data with the ‘0’ bits at the predetermined control points to thefirst delay means 44AB and an input of the first exclusive NOR means45A. The second bit insertion means 42B provides the encoded data streamcomprising ‘1’ bits at the predetermined control points to the firstNRZI coder 43B which encodes the data and provides the resulting NRZIencoded data based on the data with the ‘1’ bits at the predeterminedcontrol points to the second delay means 44B and the input of a secondexclusive NOR means 45B. The first delay means 44A delays the datacoming from the first NRZI coder 43A for the duration of one track andprovides the delayed data to the third delay means 47A and to a firstinput 48A of the selection means 48. The second delay means 44B delaysthe data coming from the second NRZI coder 43B for the duration of onetrack. and provides the delayed data to the fourth delay means 47B andto a second input 48B of the selection means 48. The third delay means47A delays the delayed data coming from the first delay means 44A by theduration of a track and provides the data, which is now delayed by theduration of two tracks compared to the output of the first NRZI coder43A, to the second input of the first exclusive NOR means 45A.

The fourth delay means 47B delays the delayed data coming from thesecond delay means 44B by the duration of a track and provides the data,which is now delayed by the duration of two tracks compared to theoutput of the second NRZI coder 43B, to the second input of the secondexclusive NOR means 45A. The output of the first exclusive NOR means 45Ais provided to the input of the first integrator means 46A, whichintegrates the output data provided by first exclusive NOR means andprovides the result of this integration to the third input 48C of theselection means 48. The output of the second exclusive NOR means 45B isprovided to the input of the second integrator means 46B, whichintegrates the output data provided by second exclusive NOR means andprovides the result of this integration to the fourth input 48D of theselection means 48. The selection means 48 determines whether thecontent of the first delay means 44A or the content of the second delaymeans 44B results in a lower crosstalk and provides the content of thatdelay means to the output of the selection means 48. The selectedcontent is provided by the output of the selection means to the output49 of the encoder 40.

The determination is done for a section of data that is present in thefirst delay means 44A and in the second delay means 44B. Once aselection is made the integrator means 46A, 46B are reset to start thedetermination for the next section of data again.

The exclusive NOR means determine the differences between the currentdata and the date that is delayed for the duration of 2 tracks. Thecurrent data corresponds with the third track 3 in FIG. 1. The data thatis delayed for the duration of two tracks corresponds to the secondtrack in FIG. 1.

The exclusive NOR means 45A, 45B thus determines the differences betweenthe second track 2 and the first track 1 in FIG. 1 for each bitposition. The third track 3 in FIG. 1 is ignored for the determinationssince it is only the victim of the crosstalk, not a contributor.

The integrators 46A, 46B effectively count the number of bit positionswhich are equal between the content of the delay means 44A, 44B and thedelayed data. A high number coming from the integrator indicates manybit positions with equal bit values. A low number coming form theintegrator indicates many bit positions with un-equal bit values. Sincethe determination is performed for both the ‘0’ value and the ‘1’ valueof the inserted bit at the predetermined control points the selectionmeans receives two indications, one from the first integrator 46A,indicating the amount of crosstalk in case a ‘0’ is inserted, and onefrom the second integrator, indicating the amount of crosstalk in case a‘1’ is inserted. By selecting the data corresponding to the integratorthat provides the lowest integrated output value, the lowest crosstalkon the record carrier is achieved.

It is to be noted that although the example uses a paralleldetermination of the inserted bit at the predetermined control pointthat yields the lowest cross talk and illustrates this example inhardware, it is equally suitable to implement this principle in a serialfashion, i.e. first determining the crosstalk for an inserted bit valueof ‘0’ and then determining the crosstalk for an inserted bit value of‘1’, then selecting the inserted bit yielding the lowest crosstalk andencoding the data using that inserted bit value for recording on therecord carrier. This can of course also be done in software on aprocessing means instead of in hardware.

It should further be noted that although the invention was illustratedusing bit insertion at predetermined control points other methods toaffect the way the data is encoded and decoded exist that can just aseasily be applied. An example of this is code word replacement whereduring the encoding some code words, based on a predetermined table, orsequences of code words are replaced by the coder 41 by other code wordsthat can never occur. The code words that can never exist affect the waythe data is encoded by the NRZI coders 43A, 43B, for instance bydiffering an odd number of ‘1’ from the replaced code word, and can thusaffect the amount of crosstalk. During decoding the decoder 91, insteadof removing inserted bits, replaces the code word that can never existwith the corresponding code word from the predetermined table in orderto restore the original data.

FIG. 5 shows a similar encoder 50 as the encoder 40 of FIG. 4 but nowmodified to guarantee that the resulting code words produced by theencoder 50 comply with the channel constraints. The elements 42A, 42B,43A, 43B, 44A, 44B, 45A, 45B, 46A, 46B, 47A, 47B, 48A, 48B, and 49 inFIG. 4 correspond respectively to the elements 52A, 52B, 53A, 53B, 54A,54B, 55A, 55B, 56A, 56B, 57A, 57B, 58A, 58B and 59 in FIG. 5. The coder41 of FIG. 5 is split into two identical coders 51A, 51B because the twoversions of the data stream, one with inserted bits with bit value ‘0’at the control points and one with the inserted bits with bit value ‘1’at the control points, have to be encoded to determine which bit valueof the inserted bit at the control point yields the lowest crosstalk.

To guarantee that the resulting code words comply with the channelconstraints the bit insertion means 52A, 52B are moved to a positionbefore the first coders 51A,51B instead of between the first coder 41and second 43A, 43B coders of FIG. 4. When inserting a bit into theencoded data stream, as shown in FIG. 4 where the bits are insertedafter the first coder 41, the channel constraint can be violated. Whenthe bits are inserted at predetermined control points in the data streambefore the coders 51A, 51B where the data stream is not yet encoded theinserted bits are included in the encoding. All code words produced bythe coders 51A, 51B comply with the channel constraints. The code wordsof the encoder 50 as shown in FIG. 5 therefore also comply with thechannel constraints.

FIG. 6 shows the steps of a software implementation of the invention. Ablock of data is taken from the input stream. The block of data islocated between two control points.

Next, two operations are to be performed, either serially or inparallel. First a value is chosen for the control point and the datablock from that control point until the next control point is encoded.The resulting bits are compared to the bits at corresponding positionsin the track before the previous track. This is achieved by performing abit wise exclusive NOR operation on the encoded bits and bits located incorresponding positions in the track before the previous track, i.e.bits that are delayed by two tracks.

The exclusive NOR operation results in a ‘1’ for each position where theencoded bit and the bit located in the corresponding position in thetrack before the previous track have the same bit value, i.e. both havethe bit value ‘0’ or both have the bit value ‘1’. An integrator is usedto count the number of ‘1’s resulting from the exclusive NOR operation.By counting the number of ‘1’s an indication of the cross talk isobtained. A high number of ‘1’s means that a high level of crosstalkwill present.

A low number of ‘1’s means that a low level of crosstalk andconsequently low contribution to the noise level of the data located onthe track between the two tracks being processed.

Then a second value is chosen for the control point and the data blockfrom the control point until the next control point is again encoded butnow with a different control value. The resulting bits are compared tothe bits at corresponding positions in the track before the previoustrack. This is achieved by performing a bit wise exclusive NOR operationon the encoded bits and the corresponding bits located in the trackbefore the previous track, i.e. bits that are delayed by two tracks.

The exclusive NOR operation results in a ‘1’ for each position where theencoded bit and the bit located in the corresponding position in thetrack before the previous track have the same bit value, i.e. both havethe bit value ‘0’ or both have the bit value ‘1’.

An integrator is used to count the number of ‘1’s resulting from theexclusive NOR operation. By counting the number of ‘1’s an indication ofthe cross talk is obtained. A high number of ‘1’s means that a highlevel of crosstalk will present.

A low number of ‘1’s means that a low level of crosstalk andconsequently low contribution to the noise level of the data located onthe track between the two tracks being processed.

The results of the two integrators are compared and the value for thecontrol point resulting in an encoding resulting in the lowest of thetwo results is then chosen.

The value is assigned to the control point and the encoding is nowrepeated to yield the final data to be recorded on the record carrier.

It is to be noted that this last encoding step can be avoided by using abuffers in which both versions of the encoded data block are stored andafter comparison of the results of the integrators the version of theencoded data block corresponding to the lowest result of the integratoris read from the buffer instead of being recalculated.

FIG. 7 shows the digital sum value as calculated, by integrating theoutput of the exclusive NOR, for a data block between a first controlpoint CP1 and a second control point CP2. The digital sum value can onlyincrease since it is the integration of the number of corresponding bitpositions with equal bit values. Shown are two curves, a first curvecorresponding to the data block being encoded with a first control pointvalue CPV1, the second curve corresponding to the same data block beingencoded with a second control point value CPV2.

The first end point value EP1 is the final value of the integration atthe end of the data block when the first control point value CPV1 isused when encoding. The second end point value EP2 is the final value ofthe integration at the end of the data block when the second controlpoint value CPV2 is used when encoding.

The lowest of the two end point values EP1, EP2 is chosen to be used atthe control point CP1 at the beginning of the data block. This resultsin the encoded data block causing the lowest crosstalk in theneighboring track as explained above

FIG. 8 shows a recording device comprising the invention.

The recording device 80 comprises an encoder 50, receiving data to bestored on the record carrier from an input 83. The encoder 50 comprisesthe functionality of the encoder 50 of FIG. 5. The encoded data is thenpassed on to the bit engine 81 which processes the data and records thedata on the record carrier 82 in the regular fashion. Both the bitengine 81 and the encoder 50 are controlled by the controlling means 84,for instance a microcontroller, again in the regular fashion of arecording device.

FIG. 9 shows a playback device.

The playback device 90 comprises a bit engine 81 for retrieving therecorded data from the record carrier, processing it, and providing itto the decoder 91. The decoder reverses the encoding in the regularfashion and removes the inserted bits at the predetermined controlpoints. The original data is thus restored and the decoder 91 canprovide the decoded data where the inserted bit are removed to theoutput 92 of the playback device 90.

It should again be noted that although the invention was illustratedusing bit insertion at predetermined control points other methods toaffect the way the data is encoded and decoded exist that can just aseasily be applied. An example of this is code word replacement whereduring the encoding some code words, based on a predetermined table, arereplaced by code words or sequences of code words that can never existby the coder 41. The code words that can never exist affect the way thedata is encoded by the NRZI coders, for instance by differing an oddnumber of ‘1’ from the replaced code word, and can thus affect theamount of crosstalk. During decoding the decoder 91, instead of removinginserted bits, replaces the code word that can never exist with thecorresponding code word from the predetermined table in order to restorethe original data.

1. A method for encoding a stream of input words into a stream of codewords using a channel code for storage of the stream of code words on arecording medium comprising tracks for storage of the stream code words,said method comprising the step of: coding the stream of input wordsinto the stream of code words, characterized in that the method furthercomprises the steps of: determining a control point in the data streamof input words or the stream of code words where the data stream ofinput words or the stream of code words can be altered by an alteration;for each alteration of a group of N possible alterations, determining,between a group of code words in a first track and a group of code wordsin a second track which is adjacent to a third track which is adjacentto the first track, a crosstalk value representing the cross talkaffecting the third track corresponding to the alteration; selecting anoptimum alteration, where the optimum alteration is that alteration fromthe group of N alterations which has a lowest cross talk value; andaltering the data stream using the optimum alteration.
 2. The method forencoding a stream of input words into a stream of code words using achannel code as claimed in claim 1, characterized in that N=2.
 3. Themethod for encoding a stream of input words into a stream of code wordsusing a channel code as claimed in claim 2, characterized in that thecontrol point is a bit insertion point.
 4. The method for encoding astream of input words into a stream of code words using a channel codeas claimed in claim 1, characterized in that the control point is a codeword replacement point.
 5. The method for encoding a stream of inputwords into a stream of code words using a channel code as claimed inclaim 1, characterized in that the control point is determined in thestream of input words.
 6. The method for encoding a stream of inputwords into a stream of code words using a channel code as claimed inclaim 1, characterized in that control point is determined in the streamof code words.
 7. The method for encoding a stream of input words into astream of code words using a channel code as claimed in claim 1,characterized in that a crosstalk value is determined calculating adigital sum value of an exclusive NOR operation performed bitwise on thegroup of code words in the first track and the group of code words inthe second track.
 8. The method for encoding a stream of input wordsinto a stream of code words using a channel code as claimed in claim 1,characterized in that the group of code words in the first track islimited to a section of the first track and that the group of code wordsin the second track is limited to a section of the second track and thatthe section of the first track is aligned perpendicular to a readingdirection of the first track with the section of the second track. 9.The method for encoding a stream of input words into a stream of codewords using a channel code as claimed in claim 8, characterized in thatthe bitwise exclusive NOR function includes a weighing functionreflecting a physical distance.
 10. An encoder for encoding a stream ofinput words into a stream of code words using a channel code for arecording medium comprising tracks for storage of the stream of codewords, said encoder comprising coding means for encoding the stream ofinput words into a stream of code words, characterized in that theencoder further comprises: control point alteration means having aninput for receiving a data stream and an output connected to theencoding means where the control point alteration means is operative todetermine a control point in the data stream at the input where the datastream can be altered, and to alter the control point in accordance withan alteration instruction received on a alteration instruction input;crosstalk determination means having an input connected to an output ofthe encoding means and an output, said crosstalk determination meansbeing operative to determine a first crosstalk value for a first controlpoint alteration and a second crosstalk value for a second control pointalteration; and selection means having an input connected to the outputof the crosstalk determination means and an output connected to thealteration instruction input of the control point alteration means, saidselection means being operative to select a control point alterationcorresponding to the lowest crosstalk value of the first crosstalk valueand the second crosstalk value.
 11. The encoder as claimed in claim 10,characterized in that the crosstalk determination means is operative toprocess a group of code words in a first track of the recording mediumand a group of code words in a second track of the recording mediumwhich is adjacent to a third track of the recording medium which isadjacent to the first track of the recording mediums when determining acrosstalk value representing the cross talk affecting the third track.12. A recording device comprising the encoder as claimed in claim 10.13. A recording medium comprising tracks comprising a stream of codewords, characterized in that the stream of code words comprises a firstdata block in a first track and a control point, corresponding to thefirst data block, added to the stream of code words and inserted in thetrack, the control point having a value, where the value is based on across talk between the first data block in the first track and a seconddata block in a second track, where the second track is adjacent to athird track which is adjacent to the first track.